In the 1800's and before, coal and wood burning stoves were looking to for heating both homes and businesses. Today, as oil and gas prices rise, coal and wood burning heating apparatus are again being turned to as a heating source. Unlike the modern high efficiency gas and oil burning equipment, however, wood and coal burning stoves have generally had a relatively low efficiency because they do not completely burn and hence do not completely utilize the heat value of the fuel.
Thus, typical wood burning stoves, the most well known of which is probably the Franklin stove, though practical for their time, burn wood inefficiently. The Franklin stove, like most cast iron stoves available today, provides for updraft combustion, such as is found in a fireplace and in which the volatile gases (volatiles), which are driven off as the wood burns, are generally left unburned. The unburned volatiles remain for two reasons, first because the gases, by the time they have left the wood, are generally too cool for secondary combustion and second, because oxygen that is admitted to the stove or fireplace is usually consumed by the coals at the base of the fire mass, causing the gases to rise through an oxygen-deficient atmosphere. The loss of the hot, unburned volatile gases is a serious problem, because they represent approximately half of the total heat value of the wood. It is as though one were to run an open line of natural gas up a chimney without bothering to ignite it first. In addition, the volatile gases given off from the wood without being ignited may condense on the cool sides of long metal flue pipes and drip out as creosote which may sometimes be inadvertently and dangerously burned, in their solid creosote form, as a chimney fire.
Some modern wood burning apparatus, such as the DEFIANT and VIGILANT (DEFIANT and VIGILANT are trademarks of Vermont Catings, Inc. of Randolpha, Vermont) parlor stoves manufactured by the assignee of this application, provide increased secondary combustion, which increases the stove's overall efficiency. In addition, the DEFIANT and the VIGILANT each have a horizontal combustion configuration, which increases the completeness with which fuel is burned in the primary combustion chamber. Nevertheless, there still remains in the escaping gases, a significant amount of heat energy because the secondary combustion is not complete.
There are probably many reasons for incomplete secondary combustion of the gaseous volatiles. In co-pending application Ser. No. 837,608, filed Sept. 28, 1977 and assigned to the assignee of this application, secondary combustion has been aided by providing preheated, oxygen-containing air close to the gaseous connection between the primary and secondary combustion chambers and hence at a time when the gases are hotter and more likely to burn in an oxygen rich environment. While this appears to promote better secondary combustion, there still remains, as noted above, a significant quantity of unburned gas in the escaping flue gases, whose energy, if tapped, would provide a significant increase in the efficiency of the heating apparatus.
Therefore a primary object of the invention is a heating apparatus and method having increased combustion efficiency. Other objects of the invention are a wood burning heating apparatus and method which promotes significant secondary combustion, which provides reliable operation, and which has relatively low fuel consumption.